This invention relates to superconducting compositions, i.e., compositions offering no electrical resistance at a temperature below a critical temperature; to processes for their production and to methods for their use; and to methods for increasing the superconducting transition temperature of superconducting compositions.
Superconductivity was discovered in 1911. Historically, the first observed and most distinctive property of a superconductive material is the near total loss of electrical resistance by the material when at or below a critical temperature that is a characteristic of the material. This critical temperature is referred to as the superconducting transition temperature of the material, Tc. The criteria by which a selection of the critical temperature value is determined from a transition in the change in resistance observed is often not obvious from the literature. Many past authors have chosen the mid-point of such curve as the probable critical temperature of their idealized material, while many others have chosen to report as the critical temperature the highest temperature at which a deviation from the normal state resistivity property is observed. Hence, the literature may report differing temperatures within a narrow range as the critical or superconducting transition temperature for the same material, depending on the particular author's method for selecting Tc from the observed data.
The history of research into the superconductivity of specific materials began with the discovery in 1911 that mercury superconducts at a transition temperature of about 4°K. In the late 1920's, NbC was found to superconduct at a higher temperature, namely up to about 10.5°K. Thereafter other compounds and alloys of Nb were examined and various Nb compositions were discovered with progressively, but only slightly higher, superconducting transition temperatures. In the early 1940's NbN was observed with a transition temperature of about 14°K; Nb3Sn was reported in the early 1950's; Nb3 (Al—Ge) was reported in the late 1960's; and Nb3Ge was reported in the early 1970's to have a transition temperature of about 17°K. Careful optimization of Nb3Ge thin films led to an increase of the critical temperature for such material up to 23.3°K. While this work led to progress the maximum temperature at which superconductivity could occur was raised to only 23.3°K since research started three-quarters of a century ago. The existing theories explained the superconductivity of these materials, but did not predict superconductivity of higher than 40°K. Significant progress in finding materials which superconduct at higher transition temperatures than that of Nb3Ge thin films was not made until 1986.
In 1986, specially prepared coprecipitated and heat treated mixtures of lanthanum, barium, copper and oxygen, that have an abrupt decrease in resistivity “reminiscent of the onset of percolative superconductivity” were reported by J. G. Bednorz and R. A. Muller, “Possible High Tc Superconductivity In The Ba—La—Cu—O System,” Z. Phys. B.—Condensed Matter, 64, pp. 189–193 (1986). Under atmospheric pressure conditions, the abrupt change in resistivity for these compositions—i.e., that temperature at which a portion of the material begins to show properties reminiscent of percolative superconductivity—were reported to approach the 30°K range. The authors refer to this phenomenon as a “possible” case of superconductivity. The compositions reported by Bednorz et al to have such properties at a temperature as high as 30° K comprise La5−xBaxCu5O5(3−y) where X=0.75 to 1 and Y>0. The Bednorz et al compositions are prepared by coprecipitation of Ba-, La- and Cu-nitrate solutions by addition to an oxalic acid solution followed by decomposition and solid-state reaction of the coprecipitate at 900° C. for 5 hours. Thereafter, the composition is pressed to pellets at 4 kilobars and the pellets are sintered at a temperature below 950° C. in a reduced oxygen atmosphere of 0.2×10−4 bar. Bednorz et al reported that this method of sample preparation is of crucial importance to obtaining the observed phenomena.
Superconductivity is a potentially very useful phenomenon. It reduces heat losses to zero in electrical power transmission, magnets, levitated monorail trains and many other modern devices. However, superconductivity of a material occurs only at very low temperatures. Originally, and until the inventions outlined herein, liquid helium was the required coolant to provide the conditions necessary for superconductivity to occur.
It would be desirable to produce a superconducting composition that has a transition temperature which exceeds those of superconducting compositions previously described. It would be particularly desirable to develop a superconducting composition that has the potential of having a Tc of 77°K or higher. Such a composition would enable the use of liquid nitrogen instead of liquid helium to cool the superconducting equipment and would dramatically decrease the cost of operating and insulating superconducting equipment and material.